1. Field of the Invention
The invention relates to a scanning device for optically scanning an object, which device comprises a radiation source supplying a scanning beam, a mirror objective for focusing the scanning beam to a scanning spot on such object and a radiation-sensitive detection system for receiving radiation from the object as it is scanned. The invention also relates to a mirror objective suitable for use in such a scanning device and to an optical write and/or read apparatus which includes such a device.
The object to be scanned may be the information plane of an optical record carrier. This information plane may have prerecorded information which is read by the scanning device. It is alternatively possible to write the information plane with a scanning beam which is intensity modulated in accordance with the information to be written. The object may alternatively be an arbitrarily selected object whose structure must be examined, for example by means of a scanning microscope of which the scanning device forms part.
2. Description of the Related Art
A scanning device as described in the opening paragraph, intended for reading an optical record carrier, is known from British Patent Specification No. 1,541,596. This Specification describes an optical scanning device which includes an objective comprising two mirrors whose reflective sides face each other. The one mirror is concave and its reflective side faces the record carrier. The other mirror, which is much smaller, is convex and its reflective side faces the radiation source. The radiation from the radiation source reaches the convex mirror via a passage in the larger concave mirror. The radiation is subsequently reflected towards the concave mirror and focused by this mirror to a radiation spot in the information plane of the record carrier.
The known objective forms part of a two-stage focusing system in which the entire mirror system is suspended in a magnetic coil by which the position of the objective with respect to the record carrier is controlled. The small mirror is mounted on a piece of piezoelectric material and can be moved up and down with the aid thereof so that a small extent of defocus is periodically produced in the information plane, which defocus is detected by a detection device and from which subsequently a focus-error signal is derived which is used for readjusting the position of the objective by means of the magnetic coil.
By using two separate mirrors in a close working relationship with each other, the known scanning device has a rather complicated structure and is sensitive to mechanical perturbations.